Gated filter circuit

ABSTRACT

The system is of the type that comprises a Gunn oscillator, transmitting and receiving antennas, and a mixer circuit. If there is intruder motion the doppler signal from the mixer circuit passes by way of a filter circuit and integrator to an output alarm circuit. In one embodiment the filter circuit is a notch filter that exhibits infinite attenuation above a predetermined frequency, such as 120 hertz, while passing lower intruder frequency signals. The system is also provided with novel supervisory alarm circuitry.

United States Patent Solomon Dec. 9, 1975 GATED FILTER CIRCUIT 3,497,816 2/1970 Fritz 328/138 3,539,827 11/1970 Crowe 307/233 [75] Invent El'as Mass" 3,585,400 6/1971 Brayton 307/233- 7 Assignee; G lf & western M f t i 3,705,417 12/1972 Asmussen 328/140 3,813,669 5/1974 Saunders 307/233 Company (Systems), New York, NY.

Filed: July 19, 1973 Appl. No.: 380,583

US. Cl. 307/233 R; 307/252 F; 307/283; 328/138; 328/140; 340/258; 332/9; 343/5 PD Int. Cl. H03D 13/00; HO3K 5/20; HO3D 3/00 Field of Search 328/138, 140, 141, 120; 307/233 R, 234, 295, 283; 343/5 PD, 5 DP 5/1967 Ott 328/140 X 2/1970 Jacobson 328/140 X SIGNAL CONDITIONER Primary ExaminerStanley D. Miller, Jr. Attorney, Agent, or Firm-Wolf, Greenfield & Sacks [57] ABSTRACT The system is of the type that comprises a Gunn oscillator, transmitting and receiving antennas, and a mixer circuit. If there is intruder motion the doppler signal from the mixer circuit passes by way of a filter circuit and integrator to an output alarm circuit. In one embodiment the filter circuit is a notch filter that exhibits infinite attenuation above a predetermined frequency, such as 120 hertz, while passing lower intruder frequency signals. The system is also provided with novel supervisory alarm circuitry.

11 Claims, 7 Drawing Figures TRIGGER CIRCUITRY OUTPUT GATED FILTER CIRCUIT FIELD OF THE INVENTION The present invention is in the field of microwave alarm systems and pertains in particular to an improved filter circuit for a microwave alarm system. There is also disclosed herein improved supervisory alarm circuitry.

BACKGROUND OF THE INVENTION Microwave alarm systems utilizing the doppler change in frequency for detection of a moving object or intruder are well known. The basic system generally comprises an oscillator coupled to a transmitting antenna and a receiving antenna coupled to a mixer. The signal received by the mixer is typically amplified and passed to a notch filter for attenuating frequency signals in the 120 hertz area. In the conventional system the lower frequency signals, occasioned by intruder motion, and below 120 hertz are passed by the notch filter to an integrator and trigger circuit for establishing an alarm condition.

One of the problems associated with the known systems resides in the operation of the notch filter. This filter typically does not provide infinite attenuation and consequently, a large 120 hertz signal may still be transmitted. Also, if there is a large amplitude 120 hertz signal it may be clipped by the input amplifiers. The resultant harmonics caused by this clipping may result in an alarm condition. Moreover, if there is a small drift in the center frequency of the filter the result may be a transmission of the 120 hertz signal.

In order to overcome these problems, and in accordance with the present invention there is provided a low frequency band-pass filter of improved construction. This filter provides virtually infinite attenuation beyond a predetermined upper frequency, such as 120 hertz, while lower frequency signals are passed.

Another object of the present invention is to provide an improved microwave alarm system having either audible or visual indication means associated therewith.

A further object of the present invention is to provide an improved band-pass filter for use in a microwave alarm system that is relatively simple in construction, that can be manufactured relatively cheaply and that is characterized by improved filtering characteristics.

The prior art systems have also been provided with supervisory alarm circuitry. See for example, U.S. Pat. No. 3,697,989. The prior art generally teaches the use of either a gated supervisory signal or a continuous supervisory signal. The continuous supervision mode of operation provides for more fail-safe operation. However, when using a Gunn diode it is usually necessary to redesign the power supply for the diode to provide continuous frequency modulation.

Accordingly, another object of the present invention is to provide a microwave alarm system having continuous supervision circuitry including means for modulating the Gunn oscillator diode by modulating the current through the resistor in series with the diode by means of a parallel conduction path. In this way, the conventional power supply for the diode can still be employed.

SUMMARY OF THE INVENTION To accomplish the foregoing and other objects of this invention, there is provided an improved low pass filter.

In the disclosed embodiment this filter circuit is used in a microwave alarm system. However, this filter circuit may also be used in other systems or combinations requiring a low pass filter. Also, the principles and concepts of the filter of this invention may be applicable to other types of filtering circuits such as a highpass filter.

In the disclosed embodiment the doppler signal passes through a signal conditioner which converts the signal to a binary pulse train of like frequency. The filter includes a charging circuit that preferably comprises a resistor and capacitor connected in series. When the binary signal is at one level the capacitor is inhibited from charging and when the binary signal is at its other level the capacitor is allowed to charge. The charging circuit connects to a trigger circuit which is only operable when the charging circuit is permitted to charge to a predetermined level. If the input frequency is too high, and the period is too short, then the charging circuit does not attain the trigger level and these high frequency signals are completely attenuated by the filter circuit.

In the preferred embodiment the trigger circuit couples to an integrator which preferably includes a capacitor. There is also included an input gating circuit receiving a very low frequency signal which essentially samples the integrating capacitor generating an output pulse at the rate of the gating signal but only if the input frequency is below a predetermined frequency.

In accordance with the present invention there is also provided continuous supervision circuitry. In the disclosed embodiment the Gunn oscillator is modulated at a predetermined frequency which can be detected at the receiver to determine if the Gunn oscillator is operating properly. A resistor typically connects in series with the Gunn oscillator and in the disclosed embodiment a parallel path is provided across the resistor to modulate the current therethrough thereby modulating the frequency output of the Gunn oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a microwave alarm system of the present invention;

FIG. 2 is a circuit diagram of the improved low pass filter of this invention also shown in the block diagram of FIG. 1;

FIG. 3 shows waveforms associated with the circuit of FIG. 2;

FIG. 4 is a block diagram similar to the one shown in FIG. 1 and additionally including supervisory signal means;

FIG. 5 is a circuit diagram of the modulator and Gunn oscillator shown in FIG. 4;

FIG. 6 is a block circuit diagram of a portion of the system of FIG. 1 and showing in particular the ganged range control and sensitivity adjustment; and

FIG. 7 is circuit diagram of a portion of the circuitry shown in FIG. 2 and further including visual or audible indicator means at the output thereof.

DETAELED DESCRIPTION FIG. 1 shows a basic microwave alarm system comprising a Gunn diode oscillator 10, a transmitting antenna 12, a receiving antenna 14, and mixer 16. The Gunn diode oscillator includes a Gunn diode that transmits its characteristic frequency when biased at the correct voltage. The transmitted energy is received after it has been reflected at the receiving antenna 14 which is coupled to the mixer 16. The mixer 16 usually comprises a mixer diode.

In order to provide for the proper mixing action there is usually provided a local feedback path between the transmitter and receiver. This feedback path may be achieved by means of a boss or notch" between the antenna horns.

The signal from mixer 16, which is at a frequency corresponding to the velocity of motion in the area to be secured, passes to amplifier 20 and from there to filter circuit 24. Because there is usually a large amplitude signal in the frequency area of 120 hertz it is common to provide a filter circuit 24 that is a notch filter. The low frequency intruder signals passed by filter circuit 24 are coupled to integrator 26 and the output of the integrator is used to trigger the output alarm 30 which usually includes an alarm relay. The output alarm 30 may be of conventional well-known design.

In the diagram of FIG. 1, under quiescent conditions, there is no low frequency signal received by the mixer 16. However, when movement occurs a low frequency signal is detected by mixer 16. This low frequency signal is referred to as the doppler signal and is the difference frequency between the transmitted and received signals.

Most of the blocks shown in FIG. 1 may be considered as conventional. However, in accordance with the present invention there is provided an improved filter circuit 24 which is shown in a circuit diagram in FIG. 2. The filter circuit shown in FIG. 2 is a low pass filter and generally comprises a signal conditioner 32, a charging circuit 36, a trigger circuit 40, an integrator capacitor 44, and an output circuit 48. As previously indicated, the filter circuit shown in FIG. 2 has infinite attenuation above a predetermined frequency and passes those signals below this predetermined frequency. With the addition of appropriate gating circuitry the circuit shown in FIG. 2 could also be used as a high-pass filter.

The signal conditioner 32 includes transistors T1 and T2 and associated biasing resistors. The signal coupled from the input to the base of transistor T1 may be considered as a sinusoidal signal of frequency corresponding to the velocity of motion in the area being secured by the system. Transistor T1 has a squared output as shown in FIG. 2 (input to transistor T2) at its collector and this signal is coupled by way of resistor 33 to the base of transistor T2. Transistor T2 causes this signal to be inverted at its output collector. Thus, transistor T2 is alternately conducting or non-conducting. During the period that transistor T2 is non-conducting diode 34 is back-biased and a charge path is provided by way of resistor R to charging capacitor C.

Alternatively, when transistor T2 is conducting capacitor C is maintained discharged and diode 34 is forward-biased.

FIG. 3 shows the output signals at the collector of transistor T2 for frequencies above and below the cutoff frequency of, for example, I20 hertz. FIG. 3 also shows the charging waveforms which are linear approx- 4 imations of the voltage increase across capacitor C. When this voltage reaches the trigger voltage Vr as indicated in the low frequency signal shown in FIG. 3, then an output voltage V0 is coupled to the integrating capacitor 44.

In FIG. 2 the trigger circuit 40 includes a programmable uni-junction transistor 42 having its anode coupled to capacitor C and its cathode coupled by way of resistor 43 to ground. The gate electrode of uni-junction transistor 42 couples to potentiometer 41. Potentiometer 41 sets a reference voltage at the gate electrode of transistor 42. As capacitor C charges a point is reached at which the anode voltage of transistor 42 is sufficiently positive to cause conduction of transistor 42. When this occurs capacitor C discharges through transistor 42 and most of the current passes by way of diode 45 to integrating capacitor 44. A small amount of the discharge current also passes through resistor 43.

To adjust the filter of FIG. 2 to say hertz, a signal is applied to the base of transistor T1. A squared output waveform is obtained at the collector of transistor T2. Diode 34 is alternately conducting or blocking depending upon whether transistor T2 is on or off, respectively. When transistor T2 is on capacitor C can only charge to a low value given by the voltage at the collector of transistor T2 and the forward voltage drop of diode 34. When transistor T2 is off, capacitor C charges via resistor R to a voltage given by the equation:

where I elapsed charging time, v voltage after period t seconds, R resistance in series with charging capacitor, C value of charging capacitor, and V source voltage.

It can be seen from FIG. 3 that when the frequency of the waveform at the collector of transistor T2 is above 120 hertz then the charge on capacitor C does not attain the trigger voltage level Vr. By adjusting the value of potentiometer 41 it is therefore possible to either decrease or increase the trigger voltage Vr. If the trigger voltage is decreased, then the time period t to charge capacitor C to this lower voltage will be reduced and hence the acceptable input frequency will be higher. Alternatively, if the trigger voltage is increased, the acceptable input frequency will be lower. When there is no input signal, transistor T2 clamps capacitor C to below the trigger level and there is no output.

If there were no integrating capacitor 44 shown in FIG. 2 the frequency of the output voltage Vo would be inversely proportional to the input signal frequency as the uni-junction transistor 42 is basically operated as a gated free-running oscillator whose output frequency is dependent upon the time period of the input signal. The greater this period (lower the input frequency), frequency, the larger the number of output pulses.

However, in some applications it is desireable to not produce an increasing number of output pulses as the frequency decreases. One method of providing this type of operation is to integrate the output of the trigger circuit and periodically gate this integrated output.

The output circuit 48 shown in FIG. 2 includes transistors T3 and T4. The integrated voltage across capacitor 44 is sensed at the base of transistor T4. However, transistor T4 is not capable of conduction unless transistor T3 is also conducting. Transistor T3 has a relatively low frequency gating signal coupled to its base. This signal may typically be at a frequency of 5 hertz which is lower than any expected intruder frequency. If capacitor 44 has been charged previously transistor T4 is capable of conduction. When transistor T3 does conduct only then is there an output at the collector of transistor T4. Therefore, the output of the circuit of FIG. 2 is directly dependent upon the sampling input to the base of transistor T3, and is not dependent upon the number of pulses coupled to integrating capacitor 44. FIG. 4 shows a block diagram quite similar to the one shown in FIG. 1. In FIG. 4 like reference characters will be used where appropriate. FIG. 4 shows a Gunn diode oscillator 10 coupled to a transmitting antenna 12. The receiving antenna 14 couples to a mixer 16. FIG. 1 also shows a modulator 11 coupled to the Gunn diode oscillator 10. FIG. 5 which is discussed in more detail hereinafter shows one circuit embodiment for the modulator 11 and Gunn diode oscillator shown in FIG. 4. The modulator 11 may typically provide high frequency modulation well above the 120 hertz rate.

The rest of the circuitry shown in block form in FIG. 4 may be divided into a main channel including amplifiers 50 and 54, low pass filter 56 and integrator 60, and a supervisory channel including amplifier 62 and integrator 66. The output of integrators 60 and 66 couple to alarm circuit 70.

The amplifiers 50 and 54 have capacitors 51 and 55 associated therewith. These capacitors 51 and 55 effectively reject the high frequency modulating signal and thus the main channel is provided for detection primarily of the low frequency doppler signals which are passed by filter 56.

The supervisory channel includes a capacitor 63 which couples to amplifier 62. Capacitor 63 attenuates the low frequency signals but passes the higher frequency modulating signal. As long as this modulating signal is present there will therefore be an output from integrator 66 holding off the supervisory signal and preventing actuation of alarm 70. If the modulating signal disappears then the supervisory signal disappears and the alarm circuit 70 is actuated by way of integrator 66.

Referring now to FIG. 5 there is shown a circuit diagram of the modulator and Gunn diode oscillator shown in FIG. 4. The modulating technique disclosed in FIG. 5 is relatively simple employing a semi-conductor switch shown as transistor 72 which, when conductive, couples resistor 73 in parallel with resistor 74 which is in series with the Gunn diode. The Gunn diode will oscillate at its characteristic frequency as long as the bias provided by way of resistor 74 is maintained constant. However, when transistor 72 is conductive the effective resistance of the parallel combination of resistors 73 and 74 will modulate the oscillation of the Gunn diode.

The modulator 11 also includes a transistor 75 and a uni-junction transistor 78. The gate of transistor 78 is maintained at a reference voltage set by potentiometer 79. The transistors 75 and 78 along with capacitor 80 and the associated resistors provide a relaxation oscillator wherein capacitor 80 is cyclically charged and discharged by way of transistor 78 to cause periodic conduction of transistor 75. When transistor 75 is conducting, transistor 72 also conducts. Alternatively, when transistor 75 is not conducting transistor 72 is off. Therefore, the Gunn diode is modulated at a frequency dependent upon frequency of operation of modulator II. This frequency of operation is determined by the value of capacitor 80 and its associated resistors and can also be determined by the setting of potentiometer 79. One particular relaxation oscillator has been depicted, however, any other design may be substituted for switching transistor 72.

Another feature of the present invention is concerned with the circuit block diagram shown in FIG. 6. FIG. 6 shows a portion of the system previously discussed in FIG. 1. In FIG. 6 there is a pre-amplifier 82 which may receive an input from the mixer. The output of pre-amplifier 82 couples by way of capacitor 83 to potentiometer 84. The moveable contact of the potentiometer couples by way of capacitor 85 to amplifier 86. The output of the amplifier couples to filter circuit 87 which may of the type shown in FIG. 2. The output of filter circuit 87 couples to integrator 88 which includes transistor 89 and potentiometers 90 and 91. Integrator 88 also includes integrating capacitor 93 which couples to the output trigger circuit 94.

The potentiometer 84 controls the amplitude gain of the system and the potentiometers 90 and 91 control the integration time constant. When the amplifier section is set at its maximum gain, there is a greater chance of a false alarm. It is desireable that at this maximum setting of potentiometer 84, three steps are necessary before an alarm results. To achieve this automatically, the two potentiometers 84 and 90 are ganged so that potentiometer 90 is at its maximum setting when potetiometer 84 is at its maximum setting thus requiring the signal to persist for a greater time before capacitor 93 reaches the trigger level.

Alternatively, when amplifier gain is at its minimum there is a smaller likelihood of a spurious alarm resulting and with the ganged arrangement of potentiometers potentiometer 90 would also be at its lower setting thereby causing capacitor 93 to charge to the trigger level faster or upon the occurrence of a first step by an intruder. It follows that intermediate positions of the ganged potentiometers 84 and 90 will give corresponding integration times.

It is a further advantage, if at any ganged setting, an independent control is provided to decrease or increase the sensitivity of the system. This is provided in the embodiment shown in FIG. 6 by potentiometer 91. It may be desireable to have the detector sensitive to two or more steps at minimum gain.

As the filter only passes frequencies below a predetermined value and as these frequencies should result in an alarm condition, it is desireable to have a means for indicating transmission of these frequencies. As the output of the filter is effectively a binary signal this can be used to switch a transistor to give a visual or audible indication of the doppler frequency. A suitable arrangement is shown in the circuit diagram of FIG. 7. FIG. 7 shows the output section of the filter previously shown in FIG. 2 including transistors T3 and T4. The collector of transistor T4 couples by way of resistor 95 to transistor 96. The collector of transistor T4 may also couple to a suitable conventional output alarm circuit. The collector of transistor 96 couples to an audible or visual indicator 97 which may simple be an indicator light.

The indicator shown in FIG. 7 has the advantage of giving an indication of when movement starts being detected during installation walk tests. Thus, if the alarm is set to detect three steps, the indicator will show when detection occurs and the count of three steps can begin.

Having described a limited number of embodiments of the present invention it should now become apparent to one skilled in the art that other embodiments and modifications of the disclosed embodiments are contemplated as falling within the scope of this invention.

What is claimed is:

l. A filter circuit comprising;

means for receiving an alternating signal,

a free-running oscillator circuit including a capacitance means and means defining a reference level to which the capacitance means is chargeable,

means coupled from said receiving means to said capacitance means for controlling operation of said oscillator to clamp said capacitance means and inhibit charging thereof during a portion of the period of the alternating signal and to unclamp said capacitance means and enable charging thereof during the remainder of the period of the alternating signal,

and means coupled from said free-running oscillator for providing an output signal when the period of the alternating signal is greater than a predetermined period corresponding to a predetermined cut-off frequency, the frequency of the output signal when occuring being proportional to the frequency of the alternating signal,

wherein said means for providing an output signal includes output circuit means and means for coupling a gating signal to said output circuit means.

2. The filter circuit of claim 1 wherein said means for receiving includes a signal conditioner having a squared alternating output signal.

3. The filter circuit of claim 1 wherein said capacitance means includes a capacitor and resistance means coupled in series.

4. The filter circuit of claim I wherein said means for controlling includes a rectifier means for maintaining said capacitor discharged during the portion of the period and permitting charging of said capacitor via said resistance means during the remainder of the period.

5. A filter circuit comprising;

means for receiving an alternating signal,

a free-running oscillator circuit including a capacitance means and means defining a reference level to which the capacitance means is chargeable,

means coupled from said receiving means to said capacitance means for controlling operation of said oscillator to clamp said capacitance means and inhibit charging thereof during a portion of the period of the alternating signal and to unclamp said capacitance means and enable charging thereof during the remainder of the period of the alternating signal,

and means coupled from said free-running oscillator for providing an output signal when the period of the alternating signal is greater than a predetermined period corresponding to a predetermined cut-off frequency, the frequency of the output signal when occuring being proportional to the frequency of the alternating signal,

wherein said means for providing an output signal comprises an integrator circuit and means including a unilateral device for coupling from an output of the oscillator to the integrator circuit.

6. The filter circuit of claim 1 wherein said free-running oscillator further comprises a three terminal semiconductor device having one terminal coupling to the capacitance means and another terminal coupling to the means defining a reference level.

7. The filter circuit of claim 6 wherein said free-running oscillator further comprises means for adjusting the reference level which in turn alters the predetermined cut-off frequency.

8. The filter circuit of claim 5 wherein said integrator circuit comprises a capacitor and said unilateral device comprises a diode, said oscillator providing charging pulses via the diode to the capacitor when the input signal is below the cut-off frequency, the frequency of the charging pulses being an inverse function of the input frequency.

9. The filter circuit of claim 8 including a pair of output transistors one of which couples to said capacitor and means for coupling a low frequency gating signal to the other transistor for providing a constant frequency output signal when the input signal frequency is below the cut-off frequency.

10. A filter circuit comprising;

means for receiving an alternating signal,

a free-running oscillator circuit including a capacitance means and means defining a reference level to which the capacitance means is chargeable,

means coupled from said receiving means to said capacitance means for controlling operation of said oscillator to clamp said capacitance means and inhibit charging thereof during a portion of the period of the alternating signal and to unclamp said capacitance means and enable charging thereof during the remainder of the period of the alternating signal,

and means coupled from said free-running oscillator for providing an output signal when the period of the alternating signal is greater than a predetermined period corresponding to a predetermined cut-off frequency, the frequency of the output signal when occuring being proportional to the frequency of the alternating signal,

a gating circuit including means for receiving the output signal and means for receiving a gating signal of frequency lower than the input signal to thereby provide a constant frequency final output signal when the input signal frequency is below cut-off frequency.

11. A filter circuit comprising;

means for receiving an alternating input signal,

means responsive to the input signal for providing an intermediate signal when the input signal is above or below a predetermined cut-off frequency,

said intermediate signal having a frequency varying inversely with reference to the input frequency,

and output gating means for receiving the intermediate signal and a constant modulating frequency signal for providing a constant frequency output signal when the input frequency is not in the cut-off region. 

1. A filter circuit comprising; means for receiving an alternating signal, a free-running oscillator circuit including a capacitance means and means defining a reference level to which the capacitance means is chargeable, means coupled from said receiving means to said capacitance means for controlling operation of said oscillator to clamp said capacitance means and inhibit charging thereof during a portion of the period of the alternating signal and to unclamp said capacitance means and enable charging thereof during the remainder of the period of the alternating signal, and means coupled from said free-running oscillator for providing an output signal when the period of the alternating signal is greater than a predetermined period corresponding to a predetermined cut-off frequency, the frequency of the output signal when occuring being proportional to the frequency of the alternating signal, wherein said means for providing an output signal includes output circuit means and means for coupling a gating signal to said output circuit means.
 2. The filter circuit of claim 1 wherein said means for receiving includes a signal conditioner having a squared alternating output signal.
 3. The filter circuit of claim 1 wherein said capacitance means includes a capacitor and resistance means coupled in series.
 4. The filter circuit of claim 1 wherein said means for controlling includes a rectifier means for maintaining said capacitor discharged during the portion of the period and permitting charging of said capacitor via said resistance means during the remainder of the period.
 5. A filter circuit comprising; means for receiving an alternating sigNal, a free-running oscillator circuit including a capacitance means and means defining a reference level to which the capacitance means is chargeable, means coupled from said receiving means to said capacitance means for controlling operation of said oscillator to clamp said capacitance means and inhibit charging thereof during a portion of the period of the alternating signal and to unclamp said capacitance means and enable charging thereof during the remainder of the period of the alternating signal, and means coupled from said free-running oscillator for providing an output signal when the period of the alternating signal is greater than a predetermined period corresponding to a predetermined cut-off frequency, the frequency of the output signal when occuring being proportional to the frequency of the alternating signal, wherein said means for providing an output signal comprises an integrator circuit and means including a unilateral device for coupling from an output of the oscillator to the integrator circuit.
 6. The filter circuit of claim 1 wherein said free-running oscillator further comprises a three terminal semiconductor device having one terminal coupling to the capacitance means and another terminal coupling to the means defining a reference level.
 7. The filter circuit of claim 6 wherein said free-running oscillator further comprises means for adjusting the reference level which in turn alters the predetermined cut-off frequency.
 8. The filter circuit of claim 5 wherein said integrator circuit comprises a capacitor and said unilateral device comprises a diode, said oscillator providing charging pulses via the diode to the capacitor when the input signal is below the cut-off frequency, the frequency of the charging pulses being an inverse function of the input frequency.
 9. The filter circuit of claim 8 including a pair of output transistors one of which couples to said capacitor and means for coupling a low frequency gating signal to the other transistor for providing a constant frequency output signal when the input signal frequency is below the cut-off frequency.
 10. A filter circuit comprising; means for receiving an alternating signal, a free-running oscillator circuit including a capacitance means and means defining a reference level to which the capacitance means is chargeable, means coupled from said receiving means to said capacitance means for controlling operation of said oscillator to clamp said capacitance means and inhibit charging thereof during a portion of the period of the alternating signal and to unclamp said capacitance means and enable charging thereof during the remainder of the period of the alternating signal, and means coupled from said free-running oscillator for providing an output signal when the period of the alternating signal is greater than a predetermined period corresponding to a predetermined cut-off frequency, the frequency of the output signal when occuring being proportional to the frequency of the alternating signal, a gating circuit including means for receiving the output signal and means for receiving a gating signal of frequency lower than the input signal to thereby provide a constant frequency final output signal when the input signal frequency is below cut-off frequency.
 11. A filter circuit comprising; means for receiving an alternating input signal, means responsive to the input signal for providing an intermediate signal when the input signal is above or below a predetermined cut-off frequency, said intermediate signal having a frequency varying inversely with reference to the input frequency, and output gating means for receiving the intermediate signal and a constant modulating frequency signal for providing a constant frequency output signal when the input frequency is not in the cut-off region. 